Instructions
Open the "Wii" menu, which is located in the lower-left-hand corner of the Wii's main system menu.
Select "Data Management."
Click either "Save Data" to display all of the save data stored on the Wii, or "Wii Channels" to display all of the Wii channels on the system. Both save data and Wii channels take up memory space, so they are both useful items to delete.
Pick the data that you want to delete and use the "Erase" button to remove it. Continue to delete more items until you clear as much storage space as you need.